There are many communications systems in which multiple transmitters send signals to one receiver which must correctly recover the information from each transmitter. A common method to allow the receiver to recognize each signal is to have transmitters take turns such that each transmitter transmits a burst of information, with the transmissions scheduled such that only one signal is received at the receiver in each opportunity or time slot. This method is commonly referred to as Time Division Multiple Access or TDMA.
The TDMA technique, while allowing multiple transmitters to send signals to a single receiver, has the drawback that the receiver generally needs to receive a certain amount of information from a transmitter in order to synchronize properly. This is particularly a problem for transmitters operating on radio frequencies, where the information is modulated onto a carrier frequency, and the receiver must determine the correct carrier frequency, carrier phase, symbol clock frequency and symbol clock phase in order to correctly recover the symbols transmitted, and thus the information contained within the burst.
One solution to this problem is to transmit a preamble in the burst which allows the receiver time to determine all of the necessary parameters to allow recovery of all of the information which will follow the preamble. The preamble may be a predetermined sequence or random data, but in either case cannot contain useful information since its purpose is to train the receiver, and the output of the receiver during this training period may or may not be correct. The problem with this solution is that a long preamble will allow the receiver to train properly, but uses tine which could be instead be dedicated to the transmission of useful information.
This is particularly a problem when the bursts of information are short, since the preamble can actually be bigger than the amount of useful information, leading to a transmission efficiency of less than 50%. The solution of creating longer bursts of information has a severe drawback in that the times between bursts will necessarily become longer resulting in delays in transmission. Using long bursts could result in the average waiting period incurred by a transmitter having useful information to transmit increasing to the point where the delay affects the quality of service.
In the transmission of voice signals delay can affect echoes and make them annoyingly noticeable to the speaker, or in extreme cases the delay can be so long that the speaker and listener notice the excessive delay and can never be sure of which party is speaking. In the transmission of data signals delay can affect scheduling of resources in the network.
For all of the aforementioned reasons, it is important to have a burst mode receiver which can train quickly and which does not need a large preamble with respect to the data field.